1. Field of the Invention
The invention relates generally to a reference standard block and a method of manufacturing reference standard blocks for use in calibrating eddy current nondestructive testing probes. More particularly, the invention relates to reference standard blocks having superficial flaws that simulate fatigue cracks and a method of manufacturing such reference standard blocks.
2. Description of the Related Art
Eddy current probes have been used and accepted as a means for locating structural flaws such as fatigue cracks formed in metal structures during nondestructive testing or eddy current nondestructive evaluation (NDE). Fatigue cracks are generally of microscopic dimensions and usually do not form a void in the metal matrix in which they are found. These cracks are also generally called discontinuities, and can weaken the structural load-bearing capabilities of the metals in which they are formed. The detection of structural flaws is of particular significance to industries such as the air transportation industry, for cracks can develop in aircraft framework as a result of repeated aircraft pressurization/depressurization events, impact associated with aircraft landings, and possibly other operational procedures. Failure to detect the presence of such structural flaws and attend to their correction can present a considerable risk to the health and safety of the crew operating the aircraft, as well as to the persons and cargo carried thereby, particularly if a structural failure occurs while the aircraft is at altitude. For these reasons, testing procedures capable of detecting flaws such as cracks in aircraft framework that are not readily detectable by the unaided human eye are particularly desirable, for such procedures allow for the repair and/or replacement of components before a potentially harmful failure can occur.
Eddy current probes which are to be used to detect flaws such as fatigue cracks in structural members need to be calibrated with a specimen which simulates the type of fatigue crack anticipated to be found in the structural members under study. The reference standards which have been produced to date have many shortcomings in this regard and do not enable accurate calibration of eddy current probes for the detection of fatigue cracks and dislocations.
In general, eddy current testing works on the following principle. A high frequency alternating current is driven through a wire coil contained in the probe. This current produces an alternating magnetic field which emanates form the end of the coil. When the end of the coil is brought into proximity of a conducting material, the magnetic field induces circulating electric currents, known as eddy currents, in the material being tested. The presence of cracks or discontinuities in the test material will alter the pattern of the induced eddy currents and the associated magnetic field, thereby producing a change in the electrical parameters of the coil in the probe. These changes in electrical parameters are electronically manipulated into a useful form, such as a meter reading or a graphic display, thereby allowing for the identification and detection of flaws that may have otherwise been visually undetectable. One of the commonly measured electrical parameters is electrical impedence.
In order to optimize the accuracy of measurements obtained from eddy current nondestructive testing of structures, the eddy current device must first be calibrated to ensure that the measurement obtained during nondestructive testing can be properly interpreted. Eddy current probes are generally calibrated by using a reference standard or test block having formed therein a manufactured flaw such as a minute fissure of prescribed dimensions. A reference standard having a manufactured flaw of dimensions at least as large as the smallest flaw for which detection is sought during an inspection of a test structure is selected for probe calibration. Eddy current values as measured by a particular probe on the selected reference standard are compared to values produced by the probe on an actual flaw or fatigue crack in a part under test to provide an indication of the severity of the flaw.
Flaws of prescribed dimensions are formed in the reference standards usually through the use of milling equipment or electrical discharge machining (EDM). Reference standards in use to date have gained notoriety for being highly variable both dimensionally and in their eddy current response. At present, there are no specifications covering the production of artifact standards or blocks, and as a result there has been a proliferation of commercial and "homemade" calibration standard blocks of variable quality. The proliferation of these blocks has caused considerable confusion in the structural flaw testing field as to which calibration standard is appropriate in a given test situation.
Instruments that are sensitive to eddy current signal phase and amplitude are able to show considerable differences in phase between a relatively wide EDM notch or milled slot, and a naturally-produced fatigue crack. Saw cuts and relatively narrow EDM notches provide more reasonable flaw approximations for probe calibration incident to an inspection testing or comparatively large defects, but fatigue crack detection requires a more precise evaluation and calibration regimen, as fatigue cracks are generally of small dimensions and are therefore difficult to identify.
Naturally-produced fatigue cracks that have been identified in portions of previously studied structural members have been used to a limited extent for probe calibration purposes, but these fatigue cracks have proven to be difficult to use on a widespread, regular basis for several reasons: (1) they are difficult to reproduce; (2) the dimensions of the crack can change over time as crack tip stresses force the crack faces toward one another and into electrical contact; and (3) the precise dimensions of the crack are never actually known until the crack is broken apart and analyzed, thereby terminating further utility of the specimen as a probe calibration device.
A number of reference standards have been produced for calibrating eddy current probes and ultrasonic-vibration testing apparatus using a wide range of manufacturing techniques. For example, U.S. Pat. No. 4,203,315 discloses a reference block for nondestructive testing that includes a plurality of inserts fitted within a housing that defines the reference block. However, a metallurgical bond is formed between the inserts and the housing by a process that includes heat treatment. A principal limitation of this reference standard lies in the fact that the reference standard is formed with numerous substantive discontinuities by virtue of the inserts positioned within the specimen.
Another reference standard is disclosed in U.S. Pat. No. 4,704,892, which provides a control specimen having a plurality of cavities machined into a portion of the specimen. This reference, and the majority of the reference standards in the prior art, however, attempt to simulate defects in the surface of a specimen rather than flaws such as cracks which extend inwardly from the specimen surface.
In view of the foregoing limitations in the related art, it is clear that there exists a need for readily reproducible reference standard blocks that permit for the accurate and reliable calibration of eddy current probes prior to use of the probes in testing structural components for flaws such as cracks that are not detectable with the unassisted human eye.